New UUID Formats
draft-peabody-dispatch-new-uuid-format-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Authors | Brad Peabody , Kyzer R. Davis | ||
| Last updated | 2022-03-31 (Latest revision 2021-10-07) | ||
| Replaced by | draft-ietf-uuidrev-rfc4122bis, draft-ietf-uuidrev-rfc4122bis | ||
| RFC stream | (None) | ||
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| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
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draft-peabody-dispatch-new-uuid-format-03
dispatch BGP. Peabody
Internet-Draft
Updates: 4122 (if approved) K. Davis
Intended status: Standards Track 31 March 2022
Expires: 2 October 2022
New UUID Formats
draft-peabody-dispatch-new-uuid-format-03
Abstract
This document presents new Universally Unique Identifier (UUID)
formats for use in modern applications and databases.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 2 October 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 5
3. Summary of Changes . . . . . . . . . . . . . . . . . . . . . 5
3.1. changelog . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Variant and Version Fields . . . . . . . . . . . . . . . . . 7
5. New Formats . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. UUID Version 6 . . . . . . . . . . . . . . . . . . . . . 8
5.2. UUID Version 7 . . . . . . . . . . . . . . . . . . . . . 9
5.3. UUID Version 8 . . . . . . . . . . . . . . . . . . . . . 10
5.4. Max UUID . . . . . . . . . . . . . . . . . . . . . . . . 11
6. UUID Best Practices . . . . . . . . . . . . . . . . . . . . . 12
6.1. Timestamp Granularity . . . . . . . . . . . . . . . . . . 12
6.2. Monotonicity and Counters . . . . . . . . . . . . . . . . 13
6.3. Distributed UUID Generation . . . . . . . . . . . . . . . 16
6.4. Collision Resistance . . . . . . . . . . . . . . . . . . 17
6.5. Global and Local Uniqueness . . . . . . . . . . . . . . . 18
6.6. Unguessability . . . . . . . . . . . . . . . . . . . . . 18
6.7. Sorting . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.8. Opacity . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.9. DBMS and Database Considerations . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
10. Normative References . . . . . . . . . . . . . . . . . . . . 20
11. Informative References . . . . . . . . . . . . . . . . . . . 20
Appendix A. Example Code . . . . . . . . . . . . . . . . . . . . 22
A.1. Creating a UUIDv6 Value . . . . . . . . . . . . . . . . . 22
A.2. Creating a UUIDv7 Value . . . . . . . . . . . . . . . . . 23
A.3. Creating a UUIDv8 Value . . . . . . . . . . . . . . . . . 24
Appendix B. Test Vectors . . . . . . . . . . . . . . . . . . . . 24
B.1. Example of a UUIDv6 Value . . . . . . . . . . . . . . . . 25
B.2. Example of a UUIDv7 Value . . . . . . . . . . . . . . . . 26
B.3. Example of a UUIDv8 Value . . . . . . . . . . . . . . . . 26
Appendix C. Version and Variant Tables . . . . . . . . . . . . . 27
C.1. Variant 10xx Versions . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
Many things have changed in the time since UUIDs were originally
created. Modern applications have a need to create and utilize UUIDs
as the primary identifier for a variety of different items in complex
computational systems, including but not limited to database keys,
file names, machine or system names, and identifiers for event-driven
transactions.
One area UUIDs have gained popularity is as database keys. This
stems from the increasingly distributed nature of modern
applications. In such cases, "auto increment" schemes often used by
databases do not work well, as the effort required to coordinate
unique numeric identifiers across a network can easily become a
burden. The fact that UUIDs can be used to create unique, reasonably
short values in distributed systems without requiring synchronization
makes them a good alternative, but UUID versions 1-5 lack certain
other desirable characteristics:
1. Non-time-ordered UUID versions such as UUIDv4 have poor database
index locality. Meaning new values created in succession are not
close to each other in the index and thus require inserts to be
performed at random locations. The negative performance effects
of which on common structures used for this (B-tree and its
variants) can be dramatic.
2. The 100-nanosecond, Gregorian epoch used in UUIDv1 timestamps is
uncommon and difficult to represent accurately using a standard
number format such as [IEEE754].
3. Introspection/parsing is required to order by time sequence; as
opposed to being able to perform a simple byte-by-byte
comparison.
4. Privacy and network security issues arise from using a MAC
address in the node field of Version 1 UUIDs. Exposed MAC
addresses can be used as an attack surface to locate machines and
reveal various other information about such machines (minimally
manufacturer, potentially other details). Additionally, with the
advent of virtual machines and containers, MAC address uniqueness
is no longer guaranteed.
5. Many of the implementation details specified in [RFC4122] involve
trade offs that are neither possible to specify for all
applications nor necessary to produce interoperable
implementations.
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6. [RFC4122] does not distinguish between the requirements for
generation of a UUID versus an application which simply stores
one, which are often different.
Due to the aforementioned issue, many widely distributed database
applications and large application vendors have sought to solve the
problem of creating a better time-based, sortable unique identifier
for use as a database key. This has lead to numerous implementations
over the past 10+ years solving the same problem in slightly
different ways.
While preparing this specification the following 16 different
implementations were analyzed for trends in total ID length, bit
Layout, lexical formatting/encoding, timestamp type, timestamp
format, timestamp accuracy, node format/components, collision
handling and multi-timestamp tick generation sequencing.
1. [ULID] by A. Feerasta
2. [LexicalUUID] by Twitter
3. [Snowflake] by Twitter
4. [Flake] by Boundary
5. [ShardingID] by Instagram
6. [KSUID] by Segment
7. [Elasticflake] by P. Pearcy
8. [FlakeID] by T. Pawlak
9. [Sonyflake] by Sony
10. [orderedUuid] by IT. Cabrera
11. [COMBGUID] by R. Tallent
12. [SID] by A. Chilton
13. [pushID] by Google
14. [XID] by O. Poitrey
15. [ObjectID] by MongoDB
16. [CUID] by E. Elliott
An inspection of these implementations and the issues described above
has led to this document which attempts to adapt UUIDs to address
these issues.
2. Terminology
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2.2. Abbreviations
The following abbreviations are used in this document:
UUID Universally Unique Identifier [RFC4122]
CSPRNG Cryptographically Secure Pseudo-Random Number Generator
MAC Media Access Control
MSB Most Significant Bit
DBMS Database Management System
3. Summary of Changes
The following UUIDs are hereby introduced:
UUID version 6 (UUIDv6)
A re-ordering of UUID version 1 so it is sortable as an opaque
sequence of bytes. Easy to implement given an existing UUIDv1
implementation. See Section 5.1
UUID version 7 (UUIDv7)
An entirely new time-based UUID bit layout sourced from the widely
implemented and well known Unix Epoch timestamp source. See
Section 5.2
UUID version 8 (UUIDv8)
A free-form UUID format which has no explicit requirements except
maintaining backward compatibility. See Section 5.3
Max UUID
A specialized UUID which is the inverse of [RFC4122],
Section 4.1.7 See Section 5.4
3.1. changelog
RFC EDITOR PLEASE DELETE THIS SECTION.
draft-03
- Reworked the draft body to make the content more concise
- UUIDv6 section reworked to just the reorder of the timestamp
- UUIDv7 changed to simplify timestamp mechanism to just
millisecond Unix timestamp
- UUIDv8 relaxed to be custom in all elements except version and
variant
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- Introduced Max UUID.
- Added C code samples in Appendix.
- Added test vectors in Appendix.
- Version and Variant section combined into one section.
- Changed from pseudo-random number generators to
cryptographically secure pseudo-random number generator (CSPRNG).
- Combined redundant topics from all UUIDs into sections such as
Timestamp granularity, Monotonicity and Counters, Collision
Resistance, Sorting, and Unguessability, etc.
- Split Encoding and Storage into Opacity and DBMS and Database
Considerations
- Reworked Global Uniqueness under new section Global and Local
Uniqueness
- Node verbiage only used in UUIDv6 all others reference random/
rand instead
- Clock sequence verbiage changed simply to counter in any section
other than UUIDv6
- Added Abbreviations section
- Updated IETF Draft XML Layout
- Added information about little-endian UUIDs
draft-02
- Added Changelog
- Fixed misc. grammatical errors
- Fixed section numbering issue
- Fixed some UUIDvX reference issues
- Changed all instances of "motonic" to "monotonic"
- Changed all instances of "#-bit" to "# bit"
- Changed "proceeding" verbiage to "after" in section 7
- Added details on how to pad 32 bit Unix timestamp to 36 bits in
UUIDv7
- Added details on how to truncate 64 bit Unix timestamp to 36
bits in UUIDv7
- Added forward reference and bullet to UUIDv8 if truncating 64
bit Unix Epoch is not an option.
- Fixed bad reference to non-existent "time_or_node" in section
4.5.4
draft-01
- Complete rewrite of entire document.
- The format, flow and verbiage used in the specification has been
reworked to mirror the original RFC 4122 and current IETF
standards.
- Removed the topics of UUID length modification, alternate UUID
text formats, and alternate UUID encoding techniques.
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- Research into 16 different historical and current
implementations of time-based universal identifiers was completed
at the end of 2020 in attempt to identify trends which have
directly influenced design decisions in this draft document
(https://github.com/uuid6/uuid6-ietf-draft/tree/master/research)
- Prototype implementation have been completed for UUIDv6, UUIDv7,
and UUIDv8 in various languages by many GitHub community members.
(https://github.com/uuid6/prototypes)
4. Variant and Version Fields
The variant bits utilized by UUIDs in this specification remain in
the same octet as originally defined by [RFC4122], Section 4.1.1.
The next table details Variant 10xx (8/9/A/B) and the new versions
defined by this specification. A complete guide to all versions
within this variant has been includes in Appendix C.1.
+------+------+------+------+---------+---------------------------+
| Msb0 | Msb1 | Msb2 | Msb3 | Version | Description |
+------+------+------+------+---------+---------------------------+
| 0 | 1 | 1 | 0 | 6 | Reordered Gregorian time- |
| | | | | | based UUID specified in |
| | | | | | this document. |
+------+------+------+------+---------+---------------------------+
| 0 | 1 | 1 | 1 | 7 | Unix Epoch time-based |
| | | | | | UUID specified in this |
| | | | | | document. |
+------+------+------+------+---------+---------------------------+
| 1 | 0 | 0 | 0 | 8 | Reserved for custom UUID |
| | | | | | formats specified in this |
| | | | | | document |
+------+------+------+------+---------+---------------------------+
Table 1: New UUID variant 10xx (8/9/A/B) versions defined by this
specification
For UUID version 6, 7 and 8 the variant field placement from
[RFC4122] are unchanged. An example version/variant layout for
UUIDv6 follows the table where M is the version and N is the variant.
00000000-0000-6000-8000-000000000000
00000000-0000-6000-9000-000000000000
00000000-0000-6000-A000-000000000000
00000000-0000-6000-B000-000000000000
xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
Figure 1: UUIDv6 Variant Examples
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5. New Formats
The UUID format is 16 octets; the variant bits in conjunction with
the version bits described in the next section in determine finer
structure.
5.1. UUID Version 6
UUID version 6 is a field-compatible version of UUIDv1, reordered for
improved DB locality. It is expected that UUIDv6 will primarily be
used in contexts where there are existing v1 UUIDs. Systems that do
not involve legacy UUIDv1 SHOULD consider using UUIDv7 instead.
Instead of splitting the timestamp into the low, mid and high
sections from UUIDv1, UUIDv6 changes this sequence so timestamp bytes
are stored from most to least significant. That is, given a 60 bit
timestamp value as specified for UUIDv1 in [RFC4122], Section 4.1.4,
for UUIDv6, the first 48 most significant bits are stored first,
followed by the 4 bit version (same position), followed by the
remaining 12 bits of the original 60 bit timestamp.
The clock sequence bits remain unchanged from their usage and
position in [RFC4122], Section 4.1.5.
The 48 bit node SHOULD be set to a pseudo-random value however
implementations MAY choose to retain the old MAC address behavior
from [RFC4122], Section 4.1.6 and [RFC4122], Section 4.5. For more
information on MAC address usage within UUIDs see the Section 8
The format for the 16-byte, 128 bit UUIDv6 is shown in Figure 1
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time_high |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time_mid | time_low_and_version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|clk_seq_hi_res | clk_seq_low | node (0-1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| node (2-5) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: UUIDv6 Field and Bit Layout
time_high:
The most significant 32 bits of the 60 bit starting timestamp.
Occupies bits 0 through 31 (octets 0-3)
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time_mid:
The middle 16 bits of the 60 bit starting timestamp. Occupies
bits 32 through 47 (octets 4-5)
time_low_and_version:
The first four most significant bits MUST contain the UUIDv6
version (0110) while the remaining 12 bits will contain the least
significant 12 bits from the 60 bit starting timestamp. Occupies
bits 48 through 63 (octets 6-7)
clk_seq_hi_res:
The first two bits MUST be set to the UUID variant (10) The
remaining 6 bits contain the high portion of the clock sequence.
Occupies bits 64 through 71 (octet 8)
clock_seq_low:
The 8 bit low portion of the clock sequence. Occupies bits 72
through 79 (octet 9)
node:
48 bit spatially unique identifier Occupies bits 80 through 127
(octets 10-15)
With UUIDv6 the steps for splitting the timestamp into time_high and
time_mid are OPTIONAL since the 48 bits of time_high and time_mid
will remain in the same order. An extra step of splitting the first
48 bits of the timestamp into the most significant 32 bits and least
significant 16 bits proves useful when reusing an existing UUIDv1
implementation.
5.2. UUID Version 7
UUID version 7 features a time-ordered value field derived from the
widely implemented and well known Unix Epoch timestamp source, the
number of milliseconds seconds since midnight 1 Jan 1970 UTC, leap
seconds excluded. As well as improved entropy characteristics over
versions 1 or 6.
Implementations SHOULD utilize UUID version 7 over UUID version 1 and
6 if possible.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unix_ts_ms |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unix_ts_ms | ver | rand_a |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var| rand_b |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rand_b |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: UUIDv7 Field and Bit Layout
unix_ts_ms:
48 bit big-endian unsigned number of Unix epoch timestamp as per
Section 6.1.
ver:
4 bit UUIDv7 version set as per Section 4
rand_a:
12 bits pseudo-random data to provide uniqueness as per
Section 6.2 and Section 6.6.
var:
The 2 bit variant defined by Section 4.
rand_b:
The final 62 bits of pseudo-random data to provide uniqueness as
per Section 6.2 and Section 6.6.
5.3. UUID Version 8
UUID version 8 provides an RFC-compatible format for experimental or
vendor-specific use cases. The only requirement is that the variant
and version bits MUST be set as defined in Section 4. UUIDv8's
uniqueness will be implementation-specific and SHOULD NOT be assumed.
The only explicitly defined bits are the Version and Variant leaving
120 bits for implementation specific time-based UUIDs. To be clear:
UUIDv8 is not a replacement for UUIDv4 where all 122 extra bits are
filled with random data.
Some example situations in which UUIDv8 usage could occur:
* An implementation would like to embed extra information within the
UUID other than what is defined in this document.
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* An implementation has other application/language restrictions
which inhibit the use of one of the current UUIDs.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| custom_a |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| custom_a | ver | custom_b |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var| custom_c |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| custom_c |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: UUIDv8 Field and Bit Layout
custom_a:
The first 48 bits of the layout that can be filled as an
implementation sees fit.
ver:
The 4 bit version field as defined by Section 4
custom_b:
12 more bits of the layout that can be filled as an implementation
sees fit.
var:
The 2 bit variant field as defined by Section 4.
custom_c:
The final 62 bits of the layout immediatly following the var field
to be filled as an implementation sees fit.
5.4. Max UUID
The Max UUID is special form of UUID that is specified to have all
128 bits set to 1. This UUID can be thought of as the inverse of Nil
UUID defined in [RFC4122], Section 4.1.7
FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF
Figure 5: Max UUID Format
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6. UUID Best Practices
The minimum requirements for generating UUIDs are described in this
document for each version. Everything else is an implementation
detail and up to the implementer to decide what is appropriate for a
given implementation. That being said, various relevant factors are
covered below to help guide an implementer through the different
trade-offs among differing UUID implementations.
6.1. Timestamp Granularity
UUID timestamp source, precision and length was the topic of great
debate while creating this specification. As such choosing the right
timestamp for your application is a very important topic. This
section will detail some of the most common points on this topic.
Reliability:
Implementations SHOULD use the current timestamp from a reliable
source to provide values that are time-ordered and continually
increasing. Care SHOULD be taken to ensure that timestamp changes
from the environment or operating system are handled in a way that
is consistent with implementation requirements. For example, if
it is possible for the system clock to move backward due to either
manual adjustment or corrections from a time synchronization
protocol, implementations must decide how to handle such cases.
(See Altering, Fuzzing, or Smearing bullet below.)
Source:
UUID version 1 and 6 both utilize a Gregorian epoch timestamp
while UUIDv7 utilizes a Unix Epoch timestamp. If other timestamp
sources or a custom timestamp epoch are required UUIDv8 SHOULD be
leveraged.
Sub-second Precision and Accuracy:
Many levels of precision exist for timestamps: milliseconds,
microseconds, nanoseconds, and beyond. Additionally fractional
representations of sub-second precision may be desired to mix
various levels of precision in a time-ordered manner.
Furthermore, system clocks themselves have an underlying
granularity and it is frequently less than the precision offered
by the operating system. With UUID version 1 and 6,
100-nanoseconds of precision are present while UUIDv7 features
fixed millisecond level of precision within the Unix epoch that
does not exceed the granularity capable in most modern systems.
For other levels of precision UUIDv8 SHOULD be utilized.
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Length:
The length of a given timestamp directly impacts how long a given
UUID will be valid. That is, how many timestamp ticks can be
contained in a UUID before the maximum value for the timestamp
field is reached. Care should be given to ensure that the proper
length is selected for a given timestamp. UUID version 1 and 6
utilize a 60 bit timestamp and UUIDv7 features a 48 bit timestamp.
Altering, Fuzzing, or Smearing:
Implementations MAY alter the actual timestamp. Some examples
included security considerations around providing a real clock
value within a UUID, to correct inaccurate clocks or to handle
leap seconds. This specification makes no requirement or
guarantee about how close the clock value needs to be to actual
time.
Padding:
When timestamp padding is required, implementations MUST pad the
most significant bits (left-most) bits with zeros. An example is
padding the most significant, left-most bits of a 32 bit Unix
timestamp with zero's to fill out the 48 bit timestamp in UUIDv7.
Truncating:
Similarly, when timestamps need to be truncated: the lower, least
significant bits MUST be used. An example would be truncating a
64 bit Unix timestamp to the least significant, right-most 48 bits
for UUIDv7.
6.2. Monotonicity and Counters
Monotonicity is the backbone of time-based sortable UUIDs. Naturally
time-based UUIDs from this document will be monotonic due to an
embedded timestamp however implementations can guarantee additional
monotonicity via the concepts covered in this section.
Additionally, care MUST be taken to ensure UUIDs generated in batches
are also monotonic. That is, if one-thousand UUIDs are generated for
the same timestamp; there is sufficient logic for organizing the
creation order of those one-thousand UUIDs. For batch UUID creation
implementions MAY utilize a monotonic counter which SHOULD increment
for each UUID created during a given timestamp.
For single-node UUID implementations that do not need to create
batches of UUIDs, the embedded timestamp within UUID version 1, 6,
and 7 can provide sufficient monotonicity guarantees by simply
ensuring that timestamp increments before creating a new UUID. For
the topic of Distributed Nodes please refer to Section 6.3
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Implementations SHOULD choose one method for single-node UUID
implementations that require batch UUID creation.
Fixed-Length Dedicated Counter Bits (Method 1):
This references the practice of allocating a specific number of
bits in the UUID layout to the sole purpose of tallying the total
number of UUIDs created during a given UUID timestamp tick.
Positioning of a fixed bit-length counter SHOULD be immediatly
after the embedded timestamp. This promotes sortability and
allows random data generation for each counter increment. With
this method rand_a section of UUIDv7 MAY be utilized as fixed-
length dedicated counter bits. In the event more counter bits are
required the most significant, left-most, bits of rand_b MAY be
leveraged as additional counter bits.
Monotonic Random (Method 2):
With this method the random data is extended to also double as a
counter. This monotonic random can be thought of as a "randomly
seeded counter" which MUST be incremented in the least significant
position for each UUID created on a given timestamp tick.
UUIDv7's rand_b section SHOULD be utilized with this method to
handle batch UUID generation during a single timestamp tick.
The following sub-topics cover methods behind incrementing either
type of counter method:
Plus One Increment (Type A):
With this increment logic the counter method is incremented by one
for every UUID generation. When this increment method is utilized
with Fixed-Length Dedicated Counter the trailing random generated
for each new UUID can help produce unguessable UUIDs. When this
increment method is utilized with Monotonic Random Counters the
resulting values are easily guessable. Implementations that favor
unguessiblity SHOULD NOT utilize this method with the monotonic
random method.
Random Increment (Type B):
With this increment the actual increment of the counter MAY be a
random integer of any desired length larger than zero. When this
increment method is utilized with Fixed-Length Dedicated Counters
the random increments MAY deplete the counter bit space (including
any rollover guards) faster than the desired if a counter of
adequate length is not selected. When this increment method is
utilized with Monotonic Random Counters the counter ensures the
UUIDs retain the required level of unguessability characters
provided by the underlying entropy.
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The following sub-topics cover topics related solely with creating
reliable fixed-length dedicated counters:
Fixed-Length Dedicated Counter Seeding:
Implementations utilizing fixed-length counter method SHOULD
randomly initialize the counter with each new timestamp tick.
However, when the timestamp has not incremented; the counter
SHOULD be frozen and incremented via the desired increment logic.
When utilizing a randomly seeded counter alongside Method 1; the
random MAY be regenerated with each counter increment without
impacting sortability. The downside is that Method 1 is prone to
overflows if a counter of adequate length is not selected or the
random data generated leaves little room for the required number
of increments. Implementations utilizing fixed-length counter
method MAY also choose to randomly initialize a portion counter
rather than the entire counter. For example, a 24 bit counter
could have the 23 bits in least-significant, right-most, position
randomly initialized. The remaining most significant, left-most
counter bits are initialized as zero for the sole purpose of
guarding against counter rollovers.
Fixed-Length Dedicated Counter Length:
Care MUST be taken to select a counter bit-length that can
properly handle the level of timestamp precision in use. For
example, millisecond precision SHOULD require a larger counter
than a timestamp with nanosecond precision. General guidance is
that the counter SHOULD be at least 12 bits but no longer than 42
bits. Care SHOULD also be given to ensure that the counter length
selected leaves room for sufficient entropy in the random portion
of the UUID after the counter. This entropy helps improve the
unguessability characteristics of UUIDs created within the batch.
The following sub-topics cover rollover handling with either type of
counter method:
Counter Rollover Guards:
The technique from Fixed-Length Dedicated Counter Seeding which
describes allocating a segment of the fixed-length counter as a
rollover guard is also recommended and SHOULD be employed to help
mitigate counter rollover issues. This same technique can be
leveraged with Monotonic random counter methods by ensuring the
total length of a possible increment in the least significant,
right most position is less than the total length of the random
being incremented. As such the most significant, left-most, bits
can be incremented as rollover guarding.
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Counter Rollover Handling:
Counter rollovers SHOULD be handled by the application to avoid
sorting issues. The general guidance is that applications that
care about absolute monotonicity and sortability SHOULD freeze the
counter and wait for the timestamp to advance which ensures
monotonicity is not broken.
Implementations MAY use the following logic to ensure UUIDs featuring
embedded counters are monotonic in nature:
1. Compare the current timestamp against the previously stored
timestamp.
2. If the current timestamp is equal to the previous timestamp;
increment the counter according to the desired method and type.
3. If the current timestamp is greater than the previous timestamp;
re-initialize the desired counter method to the new timestamp and
generate new random bytes (if the bytes were frozen or being used
as the seed for a monotonic counter).
Implementations SHOULD check if the the currently generated UUID is
greater than the previously generated UUID. If this is not the case
then any number of things could have occurred. Such as, but not
limited to, clock rollbacks, leap second handling or counter
rollovers. Applications SHOULD embed sufficient logic to catch these
scenarios and correct the problem ensuring the next UUID generated is
greater than the previous.
6.3. Distributed UUID Generation
Some implementations MAY desire to utilize multi-node, clustered,
applications which involve two or more nodes independently generating
UUIDs that will be stored in a common location. While UUIDs already
feature sufficient entropy to ensure that the chances of collision
are low as the total number of nodes increase; so does the likelihood
of a collision. This section will detail the approaches that MAY be
utilized by multi-node UUID implementations in distributed
environments.
Centralized Registry:
With this method all nodes tasked with creating UUIDs consult a
central registry and confirm the generated value is unique. As
applications scale the communication with the central registry
could become a bottleneck and impact UUID generation in a negative
way. Utilization of shared knowledge schemes with central/global
registries is outside the scope of this specification.
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Node IDs:
With this method, a pseudo-random Node ID value is placed within
the UUID layout. This identifier helps ensure the bit-space for a
given node is unique, resulting in UUIDs that do not conflict with
any other UUID created by another node with a different node id.
Implementations that choose to leverage an embedded node id SHOULD
utilize UUIDv8. The node id SHOULD NOT be an IEEE 802 MAC address
as per Section 8. The location and bit length are left to
implementations and are outside the scope of this specification.
Furthermore, the creation and negotiation of unique node ids among
nodes is also out of scope for this specification.
Utilization of either a Centralized Registry or Node ID are not
required for implementing UUIDs in this specification. However
implementations SHOULD utilize one of the two aforementioned methods
if distributed UUID generation is a requirement.
6.4. Collision Resistance
Implementations SHOULD weigh the consequences of UUID collisions
within their application and when deciding between UUID versions that
use entropy (random) versus the other components such as Section 6.1
and Section 6.2. This is especially true for distributed node
collision resistance as defined by Section 6.3.
There are two example scenarios below which help illustrate the
varying seriousness of a collision within an application.
Low Impact
A UUID collision generated a duplicate log entry which results in
incorrect statistics derived from the data. Implementations that
are not negatively affected by collisions may continue with the
entropy and uniqueness provided by the traditional UUID format.
High Impact:
A duplicate key causes an airplane to receive the wrong course
which puts people's lives at risk. In this scenario there is no
margin for error. Collisions MUST be avoided and failure is
unacceptable. Applications dealing with this type of scenario
MUST employ as much collision resistance as possible within the
given application context.
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6.5. Global and Local Uniqueness
UUIDs created by this specification MAY be used to provide local
uniqueness guarantees. For example, ensuring UUIDs created within a
local application context are unique within a database MAY be
sufficient for some implementations where global uniqueness outside
of the application context, in other applications, or around the
world is not required.
Although true global uniqueness is impossible to guarantee without a
shared knowledge scheme; a shared knowledge scheme is not required by
UUID to provide uniqueness guarantees. Implementations MAY implement
a shared knowledge scheme introduced in Section 6.3 as they see fit
to extend the uniqueness guaranteed this specification and [RFC4122].
6.6. Unguessability
Implementations SHOULD utilize a cryptographically secure pseudo-
random number generator (CSPRNG) to provide values that are both
difficult to predict ("unguessable") and have a low likelihood of
collision ("unique"). CSPRNG ensures the best of Section 6.4 and
Section 8 are present in modern UUIDs.
Advice on generating cryptographic-quality random numbers can be
found in [RFC4086]
6.7. Sorting
UUIDv6 and UUIDv7 are designed so that implementations that require
sorting (e.g. database indexes) SHOULD sort as opaque raw bytes,
without need for parsing or introspection.
Time ordered monotonic UUIDs benefit from greater database index
locality because the new values are near each other in the index. As
a result objects are more easily clustered together for better
performance. The real-world differences in this approach of index
locality vs random data inserts can be quite large.
UUIDs formats created by this specification SHOULD be
Lexicographically sortable while in the textual representation.
UUIDs created by this specification are crafted with big-ending byte
order (network byte order) in mind. If Little-endian style is
required a custom UUID format SHOULD be created using UUIDv8.
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6.8. Opacity
UUIDs SHOULD be treated as opaque values and implementations SHOULD
NOT examine the bits in a UUID to whatever extent is possible.
However, where necessary, inspectors should refer to Section 4 for
more information on determining UUID version and variant.
6.9. DBMS and Database Considerations
For many applications, such as databases, storing UUIDs as text is
unnecessarily verbose, requiring 288 bits to represent 128 bit UUID
values. Thus, where feasible, UUIDs SHOULD be stored within database
applications as the underlying 128 bit binary value.
For other systems, UUIDs MAY be stored in binary form or as text, as
appropriate. The trade-offs to both approaches are as such:
* Storing as binary requires less space and may result in faster
data access.
* Storing as text requires more space but may require less
translation if the resulting text form is to be used after
retrieval and thus maybe simpler to implement.
DBMS vendors are encouraged to provide functionality to generate and
store UUID formats defined by this specification for use as
identifiers or left parts of identifiers such as, but not limited to,
primary keys, surrogate keys for temporal databases, foreign keys
included in polymorphic relationships, and keys for key-value pairs
in JSON columns and key-value databases. Applications using a
monolithic database may find using database-generated UUIDs (as
opposed to client-generate UUIDs) provides the best UUID
monotonicity. In addition to UUIDs, additional identifiers MAY be
used to ensure integrity and feedback.
7. IANA Considerations
This document has no IANA actions.
8. Security Considerations
MAC addresses pose inherent security risks and SHOULD not be used
within a UUID. Instead CSPRNG data SHOULD be selected from a source
with sufficient entropy to ensure guaranteed uniqueness among UUID
generation. See Section 6.6 for more information.
Timestamps embedded in the UUID do pose a very small attack surface.
The timestamp in conjunction with an embedded counter does signal the
order of creation for a given UUID and it's corresponding data but
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does not define anything about the data itself or the application as
a whole. If UUIDs are required for use with any security operation
within an application context in any shape or form then [RFC4122]
UUIDv4 SHOULD be utilized.
9. Acknowledgements
The authors gratefully acknowledge the contributions of Ben Campbell,
Ben Ramsey, Fabio Lima, Gonzalo Salgueiro, Martin Thomson, Murray S.
Kucherawy, Rick van Rein, Rob Wilton, Sean Leonard, Theodore Y.
Ts'o., Robert Kieffer, sergeyprokhorenko, LiosK As well as all of
those in the IETF community and on GitHub to who contributed to the
discussions which resulted in this document.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4122>.
11. Informative References
[LexicalUUID]
Twitter, "A Scala client for Cassandra", commit f6da4e0,
November 2012,
<https://github.com/twitter-archive/cassie>.
[Snowflake]
Twitter, "Snowflake is a network service for generating
unique ID numbers at high scale with some simple
guarantees.", Commit b3f6a3c, May 2014,
<https://github.com/twitter-
archive/snowflake/releases/tag/snowflake-2010>.
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[Flake] Boundary, "Flake: A decentralized, k-ordered id generation
service in Erlang", Commit 15c933a, February 2017,
<https://github.com/boundary/flake>.
[ShardingID]
Instagram Engineering, "Sharding & IDs at Instagram",
December 2012, <https://instagram-engineering.com/
sharding-ids-at-instagram-1cf5a71e5a5c>.
[KSUID] Segment, "K-Sortable Globally Unique IDs", Commit bf376a7,
July 2020, <https://github.com/segmentio/ksuid>.
[Elasticflake]
Pearcy, P., "Sequential UUID / Flake ID generator pulled
out of elasticsearch common", Commit dd71c21, January
2015, <https://github.com/ppearcy/elasticflake>.
[FlakeID] Pawlak, T., "Flake ID Generator", Commit fcd6a2f, April
2020, <https://github.com/T-PWK/flake-idgen>.
[Sonyflake]
Sony, "A distributed unique ID generator inspired by
Twitter's Snowflake", Commit 848d664, August 2020,
<https://github.com/sony/sonyflake>.
[orderedUuid]
Cabrera, IT., "Laravel: The mysterious "Ordered UUID"",
January 2020, <https://itnext.io/laravel-the-mysterious-
ordered-uuid-29e7500b4f8>.
[COMBGUID] Tallent, R., "Creating sequential GUIDs in C# for MSSQL or
PostgreSql", Commit 2759820, December 2020,
<https://github.com/richardtallent/RT.Comb>.
[ULID] Feerasta, A., "Universally Unique Lexicographically
Sortable Identifier", Commit d0c7170, May 2019,
<https://github.com/ulid/spec>.
[SID] Chilton, A., "sid : generate sortable identifiers",
Commit 660e947, June 2019,
<https://github.com/chilts/sid>.
[pushID] Google, "The 2^120 Ways to Ensure Unique Identifiers",
February 2015, <https://firebase.googleblog.com/2015/02/
the-2120-ways-to-ensure-unique_68.html>.
[XID] Poitrey, O., "Globally Unique ID Generator",
Commit efa678f, October 2020, <https://github.com/rs/xid>.
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[ObjectID] MongoDB, "ObjectId - MongoDB Manual",
<https://docs.mongodb.com/manual/reference/method/
ObjectId/>.
[CUID] Elliott, E., "Collision-resistant ids optimized for
horizontal scaling and performance.", Commit 215b27b,
October 2020, <https://github.com/ericelliott/cuid>.
[IEEE754] IEEE, "Collision-resistant ids optimized for horizontal
scaling and performance.", Series 754-2019, July 2019,
<https://standards.ieee.org/ieee/754/6210/>.
Appendix A. Example Code
A.1. Creating a UUIDv6 Value
This section details a function in C which converts from a UUID
version 1 to version 6:
#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>
#include <arpa/inet.h>
#include <uuid/uuid.h>
/* Converts UUID version 1 to version 6 in place. */
void uuidv1tov6(uuid_t u) {
uint64_t ut;
unsigned char *up = (unsigned char *)u;
// load ut with the first 64 bits of the UUID
ut = ((uint64_t)ntohl(*((uint32_t*)up))) << 32;
ut |= ((uint64_t)ntohl(*((uint32_t*)&up[4])));
// dance the bit-shift...
ut =
((ut >> 32) & 0x0FFF) | // 12 least significant bits
(0x6000) | // version number
((ut >> 28) & 0x0000000FFFFF0000) | // next 20 bits
((ut << 20) & 0x000FFFF000000000) | // next 16 bits
(ut << 52); // 12 most significant bits
// store back in UUID
*((uint32_t*)up) = htonl((uint32_t)(ut >> 32));
*((uint32_t*)&up[4]) = htonl((uint32_t)(ut));
}
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Figure 6: UUIDv6 Function in C
A.2. Creating a UUIDv7 Value
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <time.h>
// ...
// csprng data source
FILE *rndf;
rndf = fopen("/dev/urandom", "r");
if (rndf == 0) {
printf("fopen /dev/urandom error\n");
return 1;
}
// ...
// generate one UUIDv7E
uint8_t u[16];
struct timespec ts;
int ret;
ret = clock_gettime(CLOCK_REALTIME, &ts);
if (ret != 0) {
printf("clock_gettime error: %d\n", ret);
return 1;
}
uint64_t tms;
tms = ((uint64_t)ts.tv_sec) * 1000;
tms += ((uint64_t)ts.tv_nsec) / 1000000;
memset(u, 0, 16);
fread(&u[6], 10, 1, rndf); // fill everything after the timestamp with random bytes
*((uint64_t*)(u)) |= htonll(tms << 16); // shift time into first 48 bits and OR into place
u[8] = 0x80 | (u[8] & 0x3F); // set variant field, top two bits are 1, 0
u[6] = 0x70 | (u[6] & 0x0F); // set version field, top four bits are 0, 1, 1, 1
Figure 7: UUIDv7 Function in C
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A.3. Creating a UUIDv8 Value
UUIDv8 will vary greatly from implementation to implementation. A
good candidate use case for UUIDv8 is to embed exotic timestamps like
the one found in this example which employs approximately 0.25
milliseconds and approximately 5 microseconds per timestamp tick as a
48 bit value.
#include <stdint.h>
#include <stdio.h>
#include <time.h>
int main() {
struct timespec tp;
clock_gettime(CLOCK_REALTIME, &tp);
uint64_t timestamp = (uint64_t)tp.tv_sec << 12;
// compute 12 bit (~0.25 msec precision) fraction from nsecs
timestamp |= ((uint64_t)tp.tv_nsec << 12) / 1000000000;
printf("%08llx-%04llx\n", timestamp >> 16, timestamp & 0xFFFF);
return 0;
}
Figure 8: UUIDv8 Function in C
Appendix B. Test Vectors
Both UUIDv1 and UUIDv6 test vectors utilize the same 60 bit
timestamp: 0x1EC9414C232AB00 (138648505420000000) Tuesday, February
22, 2022 2:22:22.000000 PM GMT-05:00
Both UUIDv1 and UUIDv6 utilize the same values in clk_seq_hi_res,
clock_seq_low, and node. All of which have been generated with
random data.
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# Unix Nanosecond precision to Gregorian 100-nanosecond intervals
gregorian_100_ns = (Unix_64_bit_nanoseconds / 100) + gregorian_Unix_offset
# Gregorian to Unix Offset:
# The number of 100-ns intervals between the
# UUID epoch 1582-10-15 00:00:00 and the Unix epoch 1970-01-01 00:00:00.
# gregorian_Unix_offset = 0x01b21dd213814000 or 122192928000000000
# Unix 64 bit Nanosecond Timestamp:
# Unix NS: Tuesday, February 22, 2022 2:22:22 PM GMT-05:00
# Unix_64_bit_nanoseconds = 0x16D6320C3D4DCC00 or 1645557742000000000
# Work:
# gregorian_100_ns = (1645557742000000000 / 100) + 122192928000000000
# (138648505420000000 - 122192928000000000) * 100 = Unix_64_bit_nanoseconds
# Final:
# gregorian_100_ns = 0x1EC9414C232AB00 or 138648505420000000
# Original: 000111101100100101000001010011000010001100101010101100000000
# UUIDv1: 11000010001100101010101100000000|1001010000010100|0001|000111101100
# UUIDv6: 00011110110010010100000101001100|0010001100101010|0110|101100000000
Figure 9: Test Vector Timestamp Pseudo-code
B.1. Example of a UUIDv6 Value
----------------------------------------------
field bits value_hex
----------------------------------------------
time_low 32 0xC232AB00
time_mid 16 0x9414
time_hi_and_version 16 0x11EC
clk_seq_hi_res 8 0xB3
clock_seq_low 8 0xC8
node 48 0x9E6BDECED846
----------------------------------------------
total 128
----------------------------------------------
final_hex: C232AB00-9414-11EC-B3C8-9E6BDECED846
Figure 10: UUIDv1 Example Test Vector
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-----------------------------------------------
field bits value_hex
-----------------------------------------------
time_high 32 0x1EC9414C
time_mid 16 0x232A
time_low_and_version 16 0x6B00
clk_seq_hi_res 8 0xB3
clock_seq_low 8 0xC8
node 48 0x9E6BDECED846
-----------------------------------------------
total 128
-----------------------------------------------
final_hex: 1EC9414C-232A-6B00-B3C8-9E6BDECED846
Figure 11: UUIDv6 Example Test Vector
B.2. Example of a UUIDv7 Value
This example UUIDv7 test vector utilizes a well-known 32 bit Unix
epoch with additional millisecond precision to fill the first 48 bits
rand_a and rand_b are filled with random data.
The timestamp is Tuesday, February 22, 2022 2:22:22.00 PM GMT-05:00
represented as 0x17F21CFD130 or 1645539742000
-------------------------------
field bits value
-------------------------------
unix_ts_ms 48 0x017F21CFD130
var 4 0x7
rand_a 12 0xCC3
var 2 b10
rand_b 62 0x18C4DC0C0C07398F
-------------------------------
total 128
-------------------------------
final: 017F21CF-D130-7CC3-98C4-DC0C0C07398F
Figure 12: UUIDv7 Example Test Vector
B.3. Example of a UUIDv8 Value
This example UUIDv8 test vector utilizes a well-known 64 bit Unix
epoch with nanosecond precision, truncated to the least-significant,
right-most, bits to fill the first 48 bits through version.
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The next two segments of custom_b and custom_c are are filled with
random data.
Timestamp is Tuesday, February 22, 2022 2:22:22.000000 PM GMT-05:00
represented as 0x16D6320C3D4DCC00 or 1645557742000000000
It should be noted that this example is just to illustrate one
scenario for UUIDv8. Test vectors will likely be implementation
specific and vary greatly from this simple example.
-------------------------------
field bits value
-------------------------------
custom_a 48 0x320C3D4DCC00
ver 4 0x8
custom_b 12 0x75B
var 2 b10
custom_c 62 0xEC932D5F69181C0
-------------------------------
total 128
-------------------------------
final: 320C3D4D-CC00-875B-8EC9-32D5F69181C0
Figure 13: UUIDv8 Example Test Vector
Appendix C. Version and Variant Tables
C.1. Variant 10xx Versions
+------+------+------+------+---------+----------------------------+
| Msb0 | Msb1 | Msb2 | Msb3 | Version | Description |
+------+------+------+------+---------+----------------------------+
| 0 | 0 | 0 | 0 | 0 | Unused |
+------+------+------+------+---------+----------------------------+
| 0 | 0 | 0 | 1 | 1 | The Gregorian time-based |
| | | | | | UUID from in [RFC4122], |
| | | | | | Section 4.1.3 |
+------+------+------+------+---------+----------------------------+
| 0 | 0 | 1 | 0 | 2 | DCE Security version, with |
| | | | | | embedded POSIX UIDs from |
| | | | | | [RFC4122], Section 4.1.3 |
+------+------+------+------+---------+----------------------------+
| 0 | 0 | 1 | 1 | 3 | The name-based version |
| | | | | | specified in [RFC4122], |
| | | | | | Section 4.1.3 that uses |
| | | | | | MD5 hashing. |
+------+------+------+------+---------+----------------------------+
| 0 | 1 | 0 | 0 | 4 | The randomly or pseudo- |
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| | | | | | randomly generated version |
| | | | | | specified in [RFC4122], |
| | | | | | Section 4.1.3. |
+------+------+------+------+---------+----------------------------+
| 0 | 1 | 0 | 1 | 5 | The name-based version |
| | | | | | specified in [RFC4122], |
| | | | | | Section 4.1.3 that uses |
| | | | | | SHA-1 hashing. |
+------+------+------+------+---------+----------------------------+
| 0 | 1 | 1 | 0 | 6 | Reordered Gregorian time- |
| | | | | | based UUID specified in |
| | | | | | this document. |
+------+------+------+------+---------+----------------------------+
| 0 | 1 | 1 | 1 | 7 | Unix Epoch time-based UUID |
| | | | | | specified in this |
| | | | | | document. |
+------+------+------+------+---------+----------------------------+
| 1 | 0 | 0 | 0 | 8 | Reserved for custom UUID |
| | | | | | formats specified in this |
| | | | | | document. |
+------+------+------+------+---------+----------------------------+
| 1 | 0 | 0 | 1 | 9 | Reserved for future |
| | | | | | definition. |
+------+------+------+------+---------+----------------------------+
| 1 | 0 | 1 | 0 | 10 | Reserved for future |
| | | | | | definition. |
+------+------+------+------+---------+----------------------------+
| 1 | 0 | 1 | 1 | 11 | Reserved for future |
| | | | | | definition. |
+------+------+------+------+---------+----------------------------+
| 1 | 1 | 0 | 0 | 12 | Reserved for future |
| | | | | | definition. |
+------+------+------+------+---------+----------------------------+
| 1 | 1 | 0 | 1 | 13 | Reserved for future |
| | | | | | definition. |
+------+------+------+------+---------+----------------------------+
| 1 | 1 | 1 | 0 | 14 | Reserved for future |
| | | | | | definition. |
+------+------+------+------+---------+----------------------------+
| 1 | 1 | 1 | 1 | 15 | Reserved for future |
| | | | | | definition. |
+------+------+------+------+---------+----------------------------+
Table 2: All UUID variant 10xx (8/9/A/B) version definitions.
Authors' Addresses
Brad G. Peabody
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Email: brad@peabody.io
Kyzer R. Davis
Email: kydavis@cisco.com
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